The present invention relates to a method for producing an electrode for use at high temperatures. A ceramic slip is prepared that contains a metal oxide powder as well as a solid electrolyte powder. An electrode green compact is formed from the slip. The green compact is dried and sintered. The metal oxide is then reduced to metal, in which connection the reduction may also take place during use in, for example, a high temperature fuel cell. Stabilised zirconium dioxide for example is provided as solid electrolyte, and nickel oxide for example is provided as metal oxide.
The core component of a high temperature fuel cell (HTFC), which is characterised by the direct conversion of chemical energy (in the form of combustible gases) into electrical energy, consists of a solid electrolyte on which are mounted an anode and a cathode. The operating temperature is between 750° and 1000° C. Combustion of a combustible gas takes place at the anode side, with consumption of oxygen. The anode consists for example of a mixture of nickel and Y2O3-stabilised zirconium dioxide (YSZ) in order to ensure the necessary electrical and ionic conductivity. The structure of such anodes must be gas-permeable, but on the other hand must ensure the high electrical conductivity. To this end, despite the high porosity the Ni phase and the YSZ phase must have contact with one another. This structure should not age in operation at temperatures of 750° to 1000° C., i.e. its properties should as far as possible not change, in order to obtain an optimal energy yield.
In high temperature fuel cells Ni/YSZ cermets, which were described for the first time by the Westinghouse Company in the 1960s, are often used as anode materials. A typical mixture contains 30 vol. % of Ni referred to the total solids volume. Below this Ni content the conductivity falls by roughly four orders of magnitude [see publication: Ivers Tiffée E., Wersing W., Schieβl M., Ber. Bunsenges. Phys. Chem., 94 (1990), p. 978]. Beyond this limit no significant rise in electrical conductivity above 3500 S·cm has been found in cermets [see publication: Dees D. W., Claar T. D., Hasler T. E., Fee D. C., Mrazek F. C., Journal of Electrochem. Soc., 34 (1987), p. 1241]. Completely porous and conducting anode cermets and anode function layers, which essentially contain about 30 vol. % of Ni in addition to a solid electrolyte such as for example YSZ (8–11 mole % of stabilised zirconium dioxide) are thus known in the prior art. An electronic (Ni) conductivity and an ionic conductivity (stabilised zirconium dioxide) are thereby ensured. With this mixture the coefficient of expansion of the anode is furthermore matched to that of the electrolyte. In this connection a so-called anode function layer acts as transition zone between the actual anode and the gas-tight, oxygen ion-conducting electrolyte in the high temperature fuel cell. In particular the proportion of the three-phase boundaries of electrically conducting nickel, pore space filled with fuel gas, and oxygen ion-conducting YSZ determines the efficiency of an anode. The main task of an anode is to accept the electrons that the oxygen ions release into the gaseous phase when they leave the solid electrolyte.
In recent years numerous tests have been carried out on the ageing resistance (stability) of anode structures. It was found that, with prolonged use of the anodes at the operating temperature (750°–950° C.) the conductivity of the anodes drops sharply. After ageing of the anodes in the combustion gas mixture (Ar 4% H2:4% H2O) for 2000 hours, a drop in the effective electrical conductivity of 20–40% of the initial conductivity at room temperature was measured. This could be explained by the tendency of nickel to minimise the surface energy [see publication: Tsoga A., Naoumidis A., Nikolopoulos P., “Wettability under non reactive and reactive conditions in the system Ni/YSZ and Ni/Ti—TiO2/YSZ” NATO ASI Ser., Ser. 3 1998, 58, pp. 79–86]. When thermally activated the Ni phase has a tendency to form agglomerates in the anode. Accordingly, the initially finely divided Ni channels crack, particularly in the region of large pores. For this reason the Ni network is no longer so finely divided, some current pathways become interrupted, and there is a drop in conductivity. In addition the number of three-phase points in the anode function layer also decreases, and the efficiency of the whole high temperature fuel cell is considerably reduced.
The aforedescribed phenomenon of the drop in electrical conductivity has hitherto been observed to a greater or lesser extent, irrespective of the method used to produce the anode and/or anode function layer (calendering, coat-mix method with thermocompression, film casting and silk-screen printing as well as vacuum slip cast layers or plasma spraying layers), but in any case is regarded as economically disadvantageous.